This invention relates to bottom structures for containers, and more particularly, to improved bottom structures for plastic bottles of the type suitable for containing liquids under pressure such as carbonated beverages.
The bottling of carbonated beverages in plastic presents a number of problems, many of which arise in connection with the base or bottom structure of the bottle. As is discussed with greater particularity hereinbelow, mere duplication in plastic of tranditional glass bottom configurations is unsatisfactory because of the tendency of plastics to creep or become distorted under pressure, especially in the presence of the elevated temperatures which may be encountered during shipment and storage. Such distortion may alter the shape and dimensions of traditional bottom configurations to the extent that the level of liquid within the bottle falls below the fill line, thereby threatening customer acceptance or satisfaction, and the bottle may become a so-called rocker; that is, it may become unstable on a horizontal surface.
On the other hand, it is frequently desirable that the inner and outer shapes and dimensions of plastic bottles approximate those of glass bottles of the same capacity so that they may be handled by existing equipment and, in certain instances, assist customer identification of the particular product they contain. In any event, they should be aesthetically attractive.
A plastic bottle, when filled with a carbonated beverage and capped, must be able to withstand both the impact of falling from at least a moderate height onto a hard surface and the precipitous rise in internal pressure which accompanies the impact. While this requirement also affects selection of materials and bottle-forming techniques, it is an extremely important consideration in the contemplation of bottom design.
Finally, the optimum bottom structure is one which not only meets the foregoing criteria but which may be readily formed with an economy of material, without unduly expensive or elaborate equipment, and without intricate or additional manufacturing steps.
One of the most common bottom structures employed in glass bottles of the type intended to contain beverages under pressure is the so-called champagne bottom, the outer surface of which comprises a central concavity and a convex heel surrounding the concavity and merging therewith and with an end portion of the container sidewall. The lowermost points of the heel lie in a common plane to support the bottle in an upright position on a horizontal surface. Such a bottom configuration in the appropriate thickness may be wholly satisfactory in glass because of the rigidity of that material.
When the champagne bottom is translated to a thinwalled plastic container, however, the central concavity has a tendency to evert to under internal pressure, thereby rendering the bottle unstable on a horizontal surface. Even if outright eversion does not occur, internal pressure tends to cause the bottom structure to "roll out" or flex outwardly at the juncture of the concavity and the surrounding heel, whereby the concavity becomes shallower and the radial dimension of the heel is altered. This, in turn, causes an increase in the volume enclosed by the bottle and a corresponding lowering of the level of liquid contained.
Various expedients intended to alleviate these conditions have been proposed heretofore. Among them are the bottom structures disclosed in U.S. Pat. No. 3,468,443, issued Sept. 23, 1969 to P. Marcus. The wall of each of these prior bottom structures is shown to be of a uniform thickness no greater than that of the sidewall. In fact, that portion of the wall which defines the central concavity is described as a "web." To rigidify this web a plurality of external ribs interrupt the outer surface of the concavity and extend outwardly therefrom. The ribs are distributed in a symmetrical array, each rib extending longitudinally in the direction of the heel from an inner portion of the concavity. Even with the rigidity provided by the ribs, some degree of eversion or flexure is expected, because a further, central depression is necessary to ensure that the center of the web will remain spaced from a flat supporting surface. It is to be noted that the ribs of the prior patent intersect the outer surface of the concavity in rather abrupt angles and that the ribs are solid, the latter feature requiring a substantial amount of material. Perhaps more importantly, the solid ribs present a further problem as follows.
Plastic containers, particularly plastic bottles having narrow necks, are conveniently and economically formed by the well-known blow-molding process. Briefly, a preform or parison at an appropriate temperature is received or enclosed in a mold cavity having the form of the finished container and the preform is expanded until its outer surface conforms to the cavity surface. Expansion is effected by creating an imbalance in the respective pressures acting on the inner and outer surfaces of the parison, as by introducing a gas under pressure to the interior of the parison or by drawing a vacuum about its exterior. In the case of the solid ribs of the prior patent, the corresponding grooves in a blow-mold surface would be extremely difficult if not impossible to fill with the material of the preform.